1. Field of the Invention
The present invention relates to a display device, a display panel driver, a method for driving a display panel, and a method for supplying image data to the driver, and more specifically, to overdriving of the display panel.
2. Description of the Related Art
The overdriving is one approach for improving the response speed of liquid crystal material within a liquid crystal display panel. The overdriving is a technique for improving the response speed of a liquid crystal display panel by driving liquid crystal material with a drive voltage higher than a normal drive voltage for positive drive voltage or with a drive voltage lower than a normal drive voltage for negative drive voltage, when there is a large change in the grayscale level. FIG. 1 shows an exemplary response of liquid crystal material when not using the overdriving, and FIG. 2 shows an exemplary response of the liquid crystal material when using the overdriving. The response speed of liquid crystal material is about 20 to 30 ms for black and white display, and may exceed 100 ms for grayscale display, while the current frame frequency is about 60 Hz (that is, one frame period is 16.7 ms). Therefore, as shown in FIG. 1, the brightness of a pixel actually varies over multiple frame periods with a normal driving method, when the grayscale level is to be largely changed. On the other hand, the use of the overdriving effectively accelerates the response of the liquid crystal material and thereby to shorten the actual variation time of the brightness after the grayscale level change is required, as shown in FIG. 2.
Such overdriving is often used in large-sized liquid crystal display devices, for example, a liquid crystal television and a liquid-crystal monitor for a computer, which are required to display high-quality video images. For example, Japanese Open Laid Patent Application No. H04-365094 discloses a liquid crystal television that adopts the overdriving. FIG. 3 is a block diagram showing the circuit configuration of the liquid crystal television disclosed in this patent application. The disclosed liquid crystal television is provided with an antenna 101, a tuner 102, a TV linear circuit 103, an A/D convertor circuit 104, a sync control circuit 105, a segment electrode driver circuit 106, a common electrode driver circuit 107, a liquid crystal panel 108, an image memory 111, and a ROM 112. The image memory 111 stores image data of one frame. The ROM 112 stores an image data table corresponding to two image data inputs: one is image data of current frame and the other is image data of the previous frame read from the image memory 111. When the image data changes, optimum image data are obtained from the ROM 112 according to the direction and degree of the grayscale level change, and the liquid crystal panel 108 is driven in response to the image data read from the ROM 112.
In recent years, a demand for displaying moving images is increasing also in the portable terminals; video and TV functions are provided for portable terminals. Therefore, it is one prevalent choice to apply the overdriving to liquid crystal display devices of the portable terminals.
According to inventors' examination, however, the use of conventional overdriving may cause a problem in terms of power consumption, especially in portable devices which require low power consumption. In the conventional overdriving, overdrive processing is performed in every frame period, which involves: writing image data into the image memory 111 for storing the image data of the previous frame; reading the image data of the previous frame from the image memory 111 in order to compare it with the image data of a current frame; determining the degree of overdrive from the data stored in the ROM 112; and outputting the resultant drive data. Performing such overdrive processing in every frame period undesirably increases power consumption.